The paper, sugar, textile and mining industries all have the need to separate large masses of porous materials that require washing to remove chemicals or other impurities. Rotary vacuum filters were introduced shortly after the turn of the twentieth century to separate minerals from dirt and water in the mining industry. A drum was rotated in a vat in which the minerals and water were suspended. A vacuum was created in a downleg of the rotary vacuum filter as clean shower liquid was sprayed on the surface of the rotating drum. The waste liquid passed through the drum under force of the vacuum leaving minerals entrapped on the drum surface. Rotary vacuum filters were subsequently utilized in the pulp and paper industry to separate pulp from the cooking or process liquids. Large amounts of liquid are used to remove these chemicals. The recovery of these washing chemicals has tremendous economic and environmental impact.
The oldest shower design is to spray shower liquid onto the mat with a shower pipe having openings that are dispersed across the length of the drum. The openings supply shower liquid onto spoons, whistles (as shown in FIG. 2) or parabolic lips which distribute the shower liquid tangential to the mat. These conventional shower designs, however, all give uneven distribution of the shower liquid across the drum because of overlapping spray patterns as shown in FIG. 3. The mat is therefore unevenly washed across the width of the drum. Further, the conventional shower designs can also waste shower liquid by applying more liquid than the mat will allow to pass into the drum during the time the mat is affected by the shower, thereby resulting in clean shower liquid rolling down the mat and being lost in the vat.
Another conventional shower design, known as a "Weir", is described in U.S. Pat. No. 4,511,088 and is illustrated in FIG. 4. In the weir shower, the shower liquid falls between a pair of plates down onto the mat. The weir shower, however, is sensitive to being kept absolutely level in order to achieve a consistent flow rate across the drum. Accordingly, normal wear and tear make it difficult to keep the weir flowing evenly across the drum.
A still further convention shower design, referred to as a "Uniflow" shower, is described in U.S. Pat. No. 4,616,489 and illustrated in FIG. 5, the uniflow shower includes an enlongated chamber placed axially above the drum. The lower portion of the chamber is narrow and structured such that a head of shower liquid builds up within the chamber to create a static liquid pressure which forces shower liquid to leave the chamber through a slot along its bottom. The head within the chamber causes shower liquid to flow out of the slot and through the mat. Additionally a pair of resilient skirts are attached to the slot and extend to the mat to guide the liquid perpendicularly to the mat and skirts, thus helping to force shower liquid through the mat. Also, depending on the location at which this shower is mounted, shower liquid can collect between one of the flexible skirts and the mat surface to form a standing puddle that extends across the width of the mat to facilitate uneven washing. This shower allows a standing body of liquid to form on the surface of the mat, but problems are created by the skirts contacting the mat surface at 90 degrees. The skirts are resilient and disturb the mat when at low consistency; therefore resulting in poor washing. Moreover, clumps on the surface of the mat may sometimes be knocked back into the vat instead of being smoothed into the surface of the mat. A still further disadvantage is that the width of the pond is too small for optimum washing and this design has no adjustment for mat thickness.
U.S. Pat. No. 4,907,426 discloses a shower structure, illustrated in FIG. 6, which has a fabric hinged arrangement with a rigid retainer with a distal edge. The shower liquid comes out and flows down into a lower portion of the elongated chamber which is narrow and structured such that a head of shower liquid builds up within the chamber to create a static liquid pressure which forces shower liquid to leave the chamber through a slot along its bottom. The vertically falling shower liquid hits a fabric hinge which breaks the fall, but causes eddy currents and turbulence. This hinge is also the pivotal point of the rigid retainer with a distal edge that has to be positioned to some nominal mat thickness. If production rates change the mat thickness can decrease to a point where a gap develops between the rigid distal edge and mat allowing the pond to drain away. Also, in pulp processing, lumps occur in the formed mat that can lift the entire rigid retainer and allow a gap to form between the retainers distal edge and the mat thus allowing the pond to drain away, thereby causing operational problems. Further, this shower design also creates problems in maintaining a substantial pond depth due to the fact that the amount of incoming shower liquid is limited by the hydraulic jump caused when high velocity liquid is discharged into a low velocity (pond) region thus causing an abrupt rise (wave) in the liquid surface.
In view of the above, it is an object of the invention to provide a shower that improves the washing efficiency of a rotary atmospheric vacuum filter.